This study combines the genetical, cytological and molecular characterization of the white locus of Drosophila melanogaster. It involves comparisons of the normal gene to mutant forms that perturb the regulation of the locus. The mutations currently under investigation result from the insertion/deletion of transposable elements near the 5 feet end of the gene. The molecular nature of the changes are being determined to obtain information about the mechanisms of regulation for the locus and to learn more about the nature of the mutational events that affect regulation.